Last year, about two million American women underwent some form of medical treatment, to help them conceive a child. The American Society for Reproductive Medicine estimates that infertility is a problem for more than six million Americans or about 10 percent of the reproductive-age population. The organization is running an ad campaign, to inform teenagers and young adults that behaviors they engage in now could affect their ability to have children later. But, that campaign is making some people unhappy.

The ads are, by some accounts, provocative. And the doctors who helped design them say they're supposed to be. Each of the four billboards, which first appeared on buses in several U.S. cities last year, features an unusual-looking baby bottle, illustrating a behavior that has been linked to infertility. In one ad, the bottle is being used as an ashtray, and young people are warned that if they smoke, this could be the only use they'll ever have for such a bottle.

A second ad warns them that an unhealthy body weight, too thin or too heavy, can affect fertility. And in a third ad, there's a condom next to the bottle, because many sexually transmitted diseases are known to cause sterility.

But it's the fourth billboard that is generating the most controversy. That ad features a baby bottle shaped like an hour glass, and it warns women that as they get older, their chances of conceiving a child drop dramatically. Dr. Marcel Cedars helped design the ad. She says she wanted to focus on age, because many American women believe they can have children well into their 40's.

"I think that there's so much in the lay press about women having children later, and so much in the lay press about the miracles of modern medicine," Dr. Cedars said. "People think that the biological clock that we were so worried about 10 or 15 years ago has now been reset in some way, and it's no longer an issue."

In fact, Dr. Cedars says, it's quite unusual for a woman in her 40's to conceive, and she says many of the middle-aged women giving birth in the United States are doing so with donated eggs. She says after the age of 35, most women experience some difficulty in getting pregnant.

But the National Organization for Women, or NOW, accuses Dr. Cedars and her colleagues of trying to scare women who delay having children for the sake of their careers. The director of NOW's media office would not talk to VOA on tape, saying the organization's president, Kim Gandy, didn't want to give any more "free press" to the ad campaign.

But in an editorial published recently in the newspaper USA Today, Ms. Gandy calls the campaign "a gimmick" and says it blames individual women for a problem that is caused by many factors. Marcel Cedars says she isn't trying to blame or scare anyone.

"It's not really a matter of scaring women, it's a matter of getting information out there, so that women can have that information in order to make informed choices," Dr. Cedars said. "So that they're not making decisions with misinformation, and that, you know, whether they choose to have children, whether they choose to have children in their 30's, or wait until their 40's, those are all social decisions, but they should be made in the context of an informed decision process."

The number of American women waiting until their 30's before trying to get pregnant for the first time has more than doubled since 1972, in part because of the difficulty of juggling a full-time career and the needs of a young child. Day-care isn't subsidized by the government in the United States, and in some cities, it can cost as much as a woman might earn in the workplace.

The Family and Medical Leave Act, passed in 1993, requires businesses to give employees at least twelve weeks of unpaid leave to care for a newborn, but many women say they can't afford to go that long without being paid. Marcel Cedars these are issues that need to be addressed.

"What I think the battle from NOW should be is to fight the society and major industry to recognize that there is a biological reality for female aging, and to support women more in their childbearing decisions, as is done in most of the Western European countries, where there's much more support for maternity leave, for early childcare, and that type of thing, so that women don't have to make a choice between career and childbearing," Dr. Cedars said.

Meanwhile, the American Society for Reproductive Medicine is encountering other stumbling blocks, as it attempts to inform young people about the links between behavior and infertility. The organization recently tried to display its ads in shopping malls and movie theaters, places young people are known to frequent. But mall and theater managers rejected the ads, because it was said they lacked "entertainment value." The organization isn't sure, at this point, where it will try to display the ads next. But the group remains committed to the campaign.